1. Technical Field
Aspects of the present invention relate to lasers and particularly to a pulsed fibre laser topology for producing high power laser light with a narrow line width.
2. Description of Related Art
Ytterbium (Yb) doped fibre lasers are widely used in the wavelength range of 1030-1100 nanometer due to excellent beam quality and multi-kilowatt output power level capabilities. Such lasers have a wide variety of applications in material processing, spectroscopy, medicine, etc. Considerable efforts have been applied to extend the useable wavelength range of Yb-based fibre lasers beyond 1100 nanometer. Lasers emitting in the 1100-1200 nanometer wavelength range are of great interest for many applications in metrology, remote sensing, and medicine. There is also demand for high-brightness frequency doubled 1100-1200 nanometer lasers for producing yellow-orange sources for application in spectroscopy, laser guide star generation, and medicine, e.g. ophthalmology and dermatology applications.
There are several approaches for the achievement of emission wavelengths in the 1100-1200 nanometer range. One approach disclosed by Sinha, S. et al. (Opt. Lett. 31, 347, 2006) utilizes a property of Yb-doped silica to exhibit a gain up to 1200 nanometer. A challenge with this method is caused by strong gain competition from shorter wavelengths.
Another approach uses Raman converters. Stimulated Brillouin scattering (SBS) in continuous wave (CW) Raman converters limits the power obtained in this method to about 40 W.
In international patent application publication WO2003029851, an optical fibre device is based on the Raman effect in a holey optical fibre such that optical gain is provided at a second wavelength within the fibre.
In international patent application publication WO2007127356 and US patent application publication U.S.2006198397, a pulsed cascaded Raman laser is disclosed including a pulsed light source generating a pulsed light having an optical spectrum centered at a source wavelength and a Raman conversion fibre coupled the pulsed light source. The pulsed light traverses the nonlinear Raman conversion fibre and is converted by a cascaded Stimulated Raman Scattering process into pulsed light output corresponding to last Stokes order and having an optical spectrum centered at a first output wavelength which is longer than the source wavelength.
Emission in the required spectral range can be obtained from a new type of fibre laser based on the bismuth-doped silica glass optical fibre (E. M. Dianov et al., Quantum Electron. 35, 1083 (2005)). However, up to now the efficiency of these lasers is no higher than 20%.
Other alternatives have been proposed such as a cladding pumped Yb-doped photonic bandgap fibres for suppression of amplified spontaneous emission (ASE) at 1030 nanometer. However, complex system structure and therefore difficult fabrication conditions with the additional possibility of photodarkening, when pumped at 915-976 nanometer make such a laser impractical for industrial use.
Thus there is a need for and it would be advantageous to have a scalable, high power, narrow line width, pulsed fibre laser in the 1100-1200 nanometer range with high efficiency, high peak power, excellent beam quality, and narrow linewidth as a candidate for nonlinear frequency conversion such as by using second harmonic generation.